The present invention relates to optical fibers provided with protective polymer coatings, and particularly to glass optical fibers which include a composite coating comprising an inner layer of a silicone polymer and an outer layer of a polyimide-silicone block copolymer.
It is well known to coat glass optical fibers with protective organic coatings as the glass fibers are drawn from a glass melt or solid glass preform. Glass fibers as drawn exhibit very high tensile strength, but are substantially weakened by the development of surface flaws on the surface of the fiber. Thus the application of a protective coating to the surface of the fiber before it comes into contact with any solid surface can largely preserve the inherent high strength thereof.
A number of different coating systems have been used commercially for the production of glass optical waveguide fibers for optical telecommunications. However only two coating systems are presently in widespread commercial use. The first of these systems employs coating materials which are rapidly curable by exposure to ultraviolet light. Examples of such coatings are the UV-curable urethane acrylates, representative compositions for these coatings being described in European Patent No. EP0204160.
Another commercially utilized coating system for glass optical fibers for telecommunications employs a primary or first-applied coating of a silicone polymer and a secondary coating or jacket of an extruded thermoplastic polymer such as nylon. U.S. Pat. No. 4,114,981 describes an optical coating system of this type.
UV-curable acrylate coating systems provide acceptable service in applications over a relatively broad range of ambient temperatures, but are not sufficiently stable to withstand elevated temperatures for prolonged period of use. Silicone coatings are more commonly used in aggressive environments involving prolonged exposure to high temperatures, but these coatings have the disadvantage of high cost.
Fast-cure silicone resins of the kind used for optical fiber coating do not cure as rapidly as the commercial UV-curable resins, and are also somewhat more difficult to handle due to a tacky surface quality in the cured state. Thus silicone-coated fibers are generally coated at lower speeds, tend to pick up dust and dirt after coating, and often are difficult to unwind for cabling or other processing.
Because of these factors, silicone-coated fibers are normally overcoated by extrusion with a thermoplastic jacketing material prior to final testing and shipment. This typically involves off-line extrusion processing, and the tough jacketing materials used require proof testing of the jacketed fiber at unusually high stresses to assure detection of breaks. Both of these procedures add significant cost to the fiber.
Nevertheless, there are some applications for which fiber having high-temperature operating capability is needed, and for these applications, silicone-based coating formulations offer a clear advantage over acrylate-coated fibers. At least for these uses, a silicone coating system which could be applied rapidly and, most preferably without the need for offline jacketing and high-stress proof testing would have significant commercial importance.